1. Field of the Invention
The present invention relates to a surface acoustic wave filter element, a surface acoustic wave filter, a module and a communication device provided with balanced type terminals capable of suppressing deterioration of electric characteristics.
2. Description of the Related Art
With the development of mobile communications in recent years, there are expectations for improvement in performance and reduction in size of components used. Furthermore, balancing of semiconductor components such as ICs for the purpose of low noise characteristics is developed and balancing is also required for a surface acoustic wave filter element used in the RF stage.
As a conventional filter in the RF stage of mobile communication device, a surface acoustic wave filter is widely used. Especially, a longitudinally coupled mode surface acoustic wave filter element allows a balanced-unbalanced conversion to be easily realized.
Furthermore, with regard to miniaturization, a face down mounting technology represented by flip chip and CSP (Chip Size Package) is becoming the mainstream instead of a conventional wire bonding mounting technology.
A longitudinally coupled mode surface acoustic wave filter element having conventional balanced type input/output terminals will be explained below.
FIG. 7 shows a configuration of a longitudinally coupled mode surface acoustic wave filter element having conventional balanced type input/output terminals.
In FIG. 7, the surface acoustic wave filter element is constructed of a first, second and third inter-digital transducer electrodes (hereinafter referred to as “IDT electrodes”) 702, 703 and 704 and a first and second reflector electrodes 705 and 706 on a piezoelectric substrate 701. The electrode finger on one side of the first IDT electrode 702 is connected to one balanced type terminal 707 and the electrode finger on the other side of the first IDT electrode 702 is connected to another balanced type terminal 708.
Furthermore, the electrode fingers on one side of the second and third IDT electrodes 703 and 704 are connected to an unbalanced type terminal 709 and the electrode fingers on the other side are grounded. The above-described configuration allows a surface acoustic wave filter element provided with unbalanced-balanced type terminals to be realized.
Then, an example of a configuration where the surface acoustic wave filter element is mounted face down on a circuit board will be explained. FIG. 8A is a schematic view of a configuration of the surface acoustic wave filter element in FIG. 7 on the piezoelectric substrate.
The electrode finger on one side of the first IDT electrode 702 is connected to a first electrode pad 802 through a first wiring electrode 801. The electrode finger on the other side of the first IDT electrode 702 is connected to a second electrode pad 804 through a second wiring electrode 803.
The electrode finger on one side of the second IDT electrode 703 and the electrode finger on one side of the third IDT electrode 704 are each connected to a third electrode pad 806 through a third wiring electrode 805. The grounding electrode is omitted here.
FIG. 8B illustrates a surface layer of the circuit board on which the aforementioned surface acoustic wave filter element is mounted. A circuit board 807 is provided with a first on-circuit-board wiring electrode 808, a second on-circuit-board wiring electrode 809 and a third on-circuit-board wiring electrode 810.
The surface acoustic wave filter element shown in FIG. 8A is mounted in such a way as to face the circuit board 807. For example, a mounting method by ultrasonic thermo-compression bonding using a gold bump can be used. At this time, the first electrode pad 802 is connected to the first on-circuit-board wiring electrode 808, the second electrode pad 804 is connected to the second on-circuit-board wiring electrode 809 and the third electrode pad 806 is connected to the third on-circuit-board wiring electrode 810.
The first, second and third on-circuit-board wiring electrodes 808, 809 and 810 are led out as terminals by means of through holes, via holes or external electrodes of the circuit board, etc. In this case, the first, second and third electrodes 808, 809 and 810 are connected to one side OUT1 of the balanced type output terminal, the other side OUT2 of the balanced type output terminal and unbalanced type input terminal IN respectively, thus realizing a surface acoustic wave filter provided with the unbalanced-balanced type terminals.
Furthermore, the conventional surface acoustic wave device provides a plurality of grounding connecting conductors for connecting at least one of the grounding electrode pads of the surface acoustic wave element and at least one of grounding external connection terminals of the surface mounting package on the inner surface of the surface mounting package, and thereby improves the level of out-of-band suppression (e.g., see Japanese Patent Application Laid-Open No. 11-145772).
Furthermore, according to the conventional surface acoustic wave device, if coupling between the input terminals on the unbalanced type and the balanced type IDT terminals exists, an in-phase voltage is generated, and therefore it is recommended to suppress electrical coupling between both IDTs wherever possible, but no specific configuration thereof is disclosed (e.g., see “Transactions in 2001 on Foundation/Boundary Society Conference of Institute of Electronics, Information and Communication Engineers” (Institute of Electronics, Information and Communication Engineers, Aug. 29, 2001, pp. 283 to 284)).
The entire disclosure of Laid-open No. 11-145772 and pp. 283 to 284 are incorporated herein by reference in their entirety.
However, the aforementioned surface acoustic wave filter element or surface acoustic wave filter does not deal with causes for deterioration of a balance characteristic which is one of important electric characteristics in great detail nor defines a configuration of wiring electrodes on a piezoelectric substrate and a structure of a circuit board with the balance characteristic taken into consideration.